1. Field of the Invention
This invention relates to an improvement in the process for the production of wet process phosphoric acid by separation of dissolved substances. In a particular aspect, this invention relates to a method of separating magnesium, aluminum and fluoride ions, which are present as impurities, from wet process acid.
2. Description of the Prior Art
Wet process phosphoric acid is produced by reacting phosphate rock with sulfuric acid. Phosphate rock is principally composed of tricalcium phosphate and calcium fluoride, but large amounts of contaminating substances are always present. These include silica and metal ions such as iron, aluminium, sodium, calcium and magnesium along with smaller amounts of trace impurities. During the reaction with sulfuric acid, most of the calcium crystallizes as gypsum and is removed by filtration. However, other components form soluble salts which tend to slowly separate during concentration steps, storage and handling.
As is known, crude wet process acid is subjected to a series of purification and concentration steps until a grade having about 54% or more of P.sub.2 O.sub.5 is reached. This grade, known as merchant acid, is marketed for a variety of uses. Many of the soluble salts remain in the acid after concentration. During storage of the 54% acid, the soluble salts gradually precipitate causing difficulties in handling, shipping and use. For instance, precipitated impurities can result in slime accumulation in storage tanks and other apparatus resulting in reprocessing and cleaning costs and effective reduction in storage capacity. In the case of liquid fertilizers made from wet process phosphoric acid, problems include the clogging of spray orifices and nozzles used to apply the fertilizers, and precipitates which tie up P.sub.2 O.sub.5 in a form (i.e., citrate insoluble form) that is unavailable to plants. Accordingly, it is desirable to remove as much of the dissolved impurities as possible prior to handling.
Many workers have attempted to solve this problem. W. P. Moore et al., U.S. Pat. No. 3,642,439 discloses separating silicon as SiF.sub.4 while evaporating the acid from 35% to 51% P.sub.2 O.sub.5 and adding aluminum and fluorine ions as needed to maintain optimum ratios for the precipitation of certain complexes. During evaporation, a precipitate consisting of a magnesium-aluminium-fluoride phosphate complex forms and is separated. Unfortunately, a major drawback of the Moore et al. process is the loss of valuable phosphate values in the precipitated complex. Thus, there has been a need in the art for a method of precipitating ion impurities without also precipitating valuable phosphates.
H. E. Mills, U.S. Pat. Nos. 4,136,199 and 4,243,643, discloses removing metal ions by adding calcium and fluorine ions to cause precipitation of a type of ralstonite represented by the formula Na.sub.x MG.sub.4 Al.sub.2--x (F, OH).sub.6 H.sub.2 O. These patents claim to remove 25-70% of the magnesium and 5-25% of the aluminum impurities.
Richard N. Hill, U.S. Pat. No. 4,242,198, discloses a process using an ion-exchange resin to separate magnesium. K. L. Parks et al., U.S. Pat. No. 4,299,804, discloses adding sufficient fluoride to acid of 25-33% P.sub.2 O.sub.5 causing precipitation of compounds having a stoichiometry ranging from MgAl.sub.2 F.sub.8 to MgAlF.
A. W. Peterson, U.S. Pat. No. 4,409,194, discloses removing magnesium from super phosphoric acid containing 64-72% P.sub.2 O.sub.5 by aging the acid for up to 180 hours at a temperature of up to 180.degree. C. with intermittent agitation. These conditions cause formation of a precipitate of MgH.sub.2 P.sub.2 O.sub.7 which can be separated by filtration.
Although these processes have been successful in reducing the concentrations of undesirable ions, they all have various disadvantages such as being too costly, either in terms of energy consumption or equipment costs. Accordingly there is still a need for a better process for separating such ions.